1. Field of the Invention
This invention relates to line drivers and, more particularly, to techniques for reducing ringing and power losses in line driving systems.
2. Description of Related Art
Line reflection decreases the noise margin in high-speed digital circuits, especially line reflections induced by signal buffers driving off-chip loads.
The signal buffer acts as a line driver. After the driver causes a signal transition, the transition travels from the source (near) end of the transmission line to the load (far) end. Upon reaching the end, the signal transition is usually reflected at the far end and travels back toward the source. The reflected signal is then usually again reflected upon reaching the source back towards the load. This process continues until the cumulated losses cause the reflection to die out. The resulting voltage waveform seen at either end of the transmission line is typically an exponentially-damped oscillation, often referred to as xe2x80x9cringing.xe2x80x9d
Ringing often creates problems. It often causes the voltage on the transmission line to exceed allowable or safe levels. Therefore, circuitry connected to the line must be designed to accommodate higher voltage levels than are actually needed. The ringing can also be erroneously interpreted as a change in the state of the data on the line.
In the past, efforts have been made to match the impedance of the driver to the transmission line and/or the impedance of the transmission line to the load. If the impedance at one end or the other is perfectly matched, there would normally be no reflection.
A simple approach used to match the impedance between the transmission line and the load is to connect a resistance at the end of the transmission line to ground. This approach, however, causes additional power to: be dissipated in the resistance that is added. This is undesirable in low-power applications, such as in a VLSI pin driver used for fast chip-to-chip communication.
A simple approach for matching the impedance between the driver and the transmission line is to insert a resistance in series between the output of the driver and the input of the transmission line. Again, however, the addition of such a resistance increases power dissipation.
Another problem with line driving systems is the dissipation of power that occurs during transitions of the signal. This is particularly true when the load includes a substantial capacitive reactance, such as in a VLSI pin driver used for fast chip-to-chip communication.
In short, there is a need for a driver that drives a transmission line connected to a load which substantially reduces ringing without wasting power and which, preferably, reduces the energy that is dissipated during operation.
One object of the invention is to obviate these as well as other problems in the prior art.
Another object of the invention is to reduce ringing in transmission-line driving systems.
A still further object of the invention is to reduce ringing in transmission-line driving systems without increasing power dissipation.
A still further object of the invention is to reduce the maximum voltage-level specification of circuitry that is connected to a transmission-line driving system.
A still further object of the invention is to reduce data errors caused by transient signals in transmission-line driving systems.
A still further object of the invention is to conserve energy consumed by a transmission line and the load it drives.
These as well as still further objects of the invention are achieved by an apparatus and method that transition the input signal to a transmission line in a plurality of steps and that cause at least one of those steps to coincide with the arrival of a reflected signal back at the input of the transmission line.
In one embodiment of the invention, a signal generation system generates at least a first drive signal, second drive and third drive signal, the second drive signal having a level greater than the first drive signal, and the third drive signal having a level greater than the second drive signal. A controller is in communication with the signal generation system to cause the signal generation system to deliver the first drive signal, then second drive signal, and then third drive signal to the input of the transmission line. The third drive signal begins to be delivered to the input of the transmission line approximately when a reflection of the second drive signal from the output of the transmission line first arrives at the input to the transmission line.
In a still further embodiment of the invention, the controller also causes the signal generation system to deliver the third drive signal, then second drive signal, and then drive first drive signal to the input of the transmission line. During this phase, the first drive signal begins to be delivered to the input of the transmission line at approximately when a reflection of the second drive signal from the output of the transmission line first arrives at the input to the transmission line. In this embodiment, the signal generation system preferably includes a source of the second drive signal that includes an energy storage device, such as a capacitor. In a preferred embodiment, the capacitor receives all of its charge solely from the transmission line.
In a still further embodiment of the invention, the level of the second drive signal is approximately midway between the level of the first drive signal and the third drive signal.
In a still further embodiment of the invention, the level of the second drive signal is approximately equal to the reflected level of the previous drive signal.
In a still further embodiment of the invention, the signal generation system also generates a plurality of drive signals, in addition to the first drive signal, second drive signal and third drive signal. In a preferred embodiment, the source for each of the plurality of additional drive signals includes an energy storage device, such as a capacitor. Preferably, each capacitor receives all of its charge solely from the transmission line.
In a still further embodiment of the invention, the signal generation system includes a supply for generating each of the drive signals and a switching system that selectively connects each of the drive signals to the input of the transmission line. In this embodiment, the controller controls the switching system.
The invention also includes a process for driving a transmission line connected to a load, the transmission line having an input and an output.
In one embodiment of the process, a first drive signal, second drive signal and third drive signal is generated. The second drive signal has a level greater than the first drive signal; and the third drive signal has a level greater than the second drive signal. The first drive signal, second drive signal and then third drive signal is delivered to the input of the transmission line. The delivery of the third drive signal begins approximately when the reflection of the second drive signal from the output of the transmission line first arrives at the input to the transmission line.
In another embodiment of the process, the third drive signal, second drive signal and then first drive signal is also delivered to the input of the transmission line. In this embodiment, the first drive signal begins to be: delivered to the input of the transmission line at approximately when a reflection of the: second drive signal from the output of the transmission line first arrives at the input of the transmission line.
In a still further embodiment of the process, a source is used to provide the second drive signal and includes an energy storage device, such as a capacitor. Preferably, the capacitor receives all of its charge solely from: the transmission line.
In a still further embodiment of the process, the level of the second drive signal is approximately midway between the level of the first drive signal and the third drive signal.
In a still further embodiment of the process, the level of the second drive signal is somewhat above the midway level on the rising transition and somewhat below the midway level on the falling transition.
In a still further embodiment of the process, a plurality of drive signals are generated, in addition to the first drive signal, second drive signal and third drive signal.
Preferably, a source is used to generate each of the additional drive signals, each source including an energy storage device, such as a capacitor. Preferably, each capacitor receives all of its charge solely from the transmission line.
In a still further embodiment of the process, a supply generates each of the drive signals and a switching system selectively connects each of the drive signals to the input of the transmission line. In this embodiment, a controller controls the switching system.
In a still further embodiment of the invention, a driver drives a transmission line having an input connected to the driver and an output connected to a capacitive load. The driver includes a high-potential voltage source; a first electronically-controlled switch connected between the high-potential voltage source and the input of the transmission line; a low-potential voltage source; a second electronically-controlled switch connected between the low-potential voltage source and the, input of the transmission line; an energy storage device, such as a capacitor; a third electronically-controlled switch connected between the capacitor and the input of the transmission line; and a controller connected to the first, second and third electronically-controlled switches. The controller causes the third electronically-controlled switch to close approximately when a signal injected into the transmission line by the second electronically-controlled switch returns back to input of the transmission line. The controller also causes the first electronically-controlled switch to close approximately when a signal injected into the transmission line by the second electronically-controlled switch returns back to the input of the transmission line.